The present invention relates to a control system for an infinitely variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the line pressure in a hydraulic circuit for the transmission.
A known control system for an infinitely variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of the throttle valve of the engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The output of an engine is transmitted to the drive pulley through a clutch. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At starting of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach at set values under a driving condition, the transmission ratio starts to change (to upshift). At that time if the engine speed is kept constant, the transmission ratio is automatically and continuously reduced at a speed which is determined by line pressure, the pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio. In such a system, the speed of the changing of transmission ratio up to a desired transmission ratio can not be controlled in accordance with driving conditions. Accordingly, hunting or overshooting of the transmission ratio occurs, which causes the driveability of the vehicle to reduce. In addition, since the line pressure changes stepwisely, shocks occur at the change of transmission ratio.
Japanese Patent Laid Open No. 58-88252 discloses a control system provided with a torque motor for operating a line pressure control valve. Such a system is not practical, since a source for driving the torque motor must be provided.
Japanese Patent Laid Open No. 59-159456 discloses the system provided with a first valve for changing the direction of the transmission ratio change and a second valve for controlling the transmission ratio changing speed. By controlling the spool of the second valve, the transmission ratio changing speed is controlled. However, the system is complicated in construction, since two control valves are provided in addition to the conventional system. Further, the publication does not disclose a system for controlling the changing speed of the line pressure.